1. Field of the Art
The present invention refers to a process for manufacturing printed circuit boards with spaces existing between the conducting tracks protected by a resinous, cured insulating filler material, the upper surfaces of the tracks and of the filler material being flush.
2. Background of the Invention
In printed circuit boards intended for power circuits, the conducting tracks, which are typically of copper, have a considerably large thickness so as to allow high current intensities (in the order of several amperes) circulating through them, without becoming heated beyond a predetermined threshold. Furthermore, sufficiently large separation spaces between the tracks are necessary for preventing the occurrence of electric arcs. The tracks are conventionally obtained by chemical milling of a copper plate adhered to a dielectric substrate, and in said process, the material of the tracks is exposed on the upper part and on the side edges of the tracks, and, as a result, have the risk of undergoing corrosion. Usually, a layer of solder resistant mask is subsequently applied to the outer surface of the tracks (to facilitate later installation of components by flow soldering), which layer also acts as protection against corrosion. However in some applications, such as printed circuit boards installed in motorized vehicles, for example, there are adverse environmental conditions such as humidity, saline atmosphere and vibrations or mechanical loads, among others, greatly increasing the risk of corrosion in the exposed material of the side edges of the tracks.
The applicant has developed interconnection boxes for automotive power applications in which printed circuits having a thickness exceeding 200 microns, and typically in the order of 400 microns, are used. Thus, Spanish patents ES-A 2 021 545, ES-A-2 124 177, ES-A-2 071 540 and ES-A-2 140 310 can be mentioned.
The published international patent application WO 97/16056 sets forth a resin filler for a multilayer printed circuit board in which said resin filler is arranged in the recesses existing on the surface of the printed circuit board, or in the through holes provided for therein. As a background, this patent mentions a method disclosed in Japanese patent JP-A-63-137499, published in 1986, consisting of using as a filler material, an epoxy resin paste with a viscosity adjusted to a suitable value for application thereof by prior dilution with a solvent.
A printed circuit board applicable to power circuits is known by Spanish Utility Model ES-A-103341, comprising a dielectric laminar substrate, conducting tracks, having a thickness in the order of 400 μm, applied to said substrate, and separation spaces between said tracks, where the edges of the tracks are coated with a dielectric ink protecting them from corrosive effects in moist and/or saline environments, and increasing the dielectric coefficient between adjacent tracks. This dielectric ink also acts as a solder resistant mask to facilitate installation of components by flow soldering. The dielectric ink can be applied by spraying, by curtaining, by platen roller or by silk-screening methods, and, in reference to the drawings attached thereto, it can be seen how the dielectric ink completely fills the spaces between tracks and also covers the tracks. Although it is not indicated in this Utility Model, FIG. 2 of the drawings shows that a completely flat outer surface of the circuit is not obtained, but rather there are recesses in correspondence with the areas filled with dielectric ink.
Spanish Utility Model application ES-A 1051902, with priority of German patent application DE-10121673.4, discloses a printed circuit board made up of a dielectric laminar substrate, conducting tracks applied to said substrate and separation spaces between said tracks. The thickness of the tracks is noticeably greater than 100 μm, and said spaces between tracks are filled with a dielectric material hardened by heat or ultraviolet radiation, having the greatest possible adhesion to the tracks, to the substrate and to a solder resistant mask applied to the printed circuit board, such that this printed board has an essentially flat upper surface, without breaks or recesses.
This last document does not provide information concerning the particularities of the process for applying the filler material, and an experiment with any of the known application techniques verifies that a filler material deposit is formed on top of the tracks in addition to between the tracks. Accordingly, to obtain said essentially flat upper surface, without breaks or recesses, including flush upper surfaces of the tracks and of the filler material, it is necessary to carry out an operation to remove the filler material from the top part of the tracks. One such operation can be carried out, for example, by abrasion by means of grinding wheels, platen rollers or abrasive brushes following a step for curing or hardening the filler material by heat or ultraviolet radiation.
However, this abrasion operation affecting the outer face or view of the conducting tracts causes a temperature increase of the tracks and of the filler material, which is already relatively high as a result of said curing step, after the conclusion thereof, being able to easily exceed the glass transition temperature of the filler material. When this occurs, the filler material is degraded, its internal structure changes, and it loses some of its essential features, such as, for example, resilience and adhesive power, becoming brittle and tending to become detached from the tracks and from the substrate. On the other hand, it is advisable for the filler material to have a low glass transition temperature since the lower the temperature is, the more resilient and adherent the filler material is. This means that the temperature threshold which it should not exceed is relatively low, which contradicts the necessity to carry out an abrasion operation and the temperature increase this implies.
Said mechanical polishing process will also cause thermal fatigue of the Cu of the conducting tracks, partly weakening their adherence to the substrate.
It must also be taken into account that said temperature increase together with the increase that will occur during the later soldering of components to the tracks, and generally due to any exposure of the printed circuit to a heat source, will cause a different expansion in the filler material and in the conducting material of the tracks because of their different thermal expansion coefficients, which will cause pressures on the side edges of the tracks, which will be translated into a shear stress which will affect the layer of adhesive material normally joining the tracks to the substrate. Such stress can lead to a detachment of the tracks from the substrate during the abrasion step or at a later time.
The object of the present invention is to provide a process for manufacturing printed circuit boards with spaces between tracks protected by means of a filler material which prevents the drawbacks described above caused by excessive heating in an abrasion step, and providing good insulation resistance and dielectric strength features.
Another object of the invention is that of being able to integrate on the circuit produced, in which there is no unevenness between tracks and substrate, electronic components by means of the Surface Mount Technology (SMT), and as a result, providing a circuit in which power and electronics are combined.
Another object of the invention is that of making it possible to obtain printed circuits with a conducting track thickness exceeding 200 microns, and with a spacing between said tracks of less than the conventional spacing (as it is filled with a dielectric substance), and generally with a minimum spacing of less than 0.9 mm, which provides the possibility of mounting a greater diversity of electronic components, and among them, fine pitch type electronic components.